1. Field of the Invention
The present invention generally concerns tool holders or chucks, employing collets and more particularly concerns heavy duty chucks usable in milling machines and other machine tools which insure concentricity of the cutting tool and eliminate twisting and cutting tool movement within the chuck during operation.
2. Background of the Invention
Several types of heavy duty chucks for milling machines and other machine tools are in common use. One prior art type of machine tool, or milling, chuck employs a taper collet system. This system may either be of the conventional screw type or of the ballscrew type. Another prior art type of chuck employs a roll-rock or a roll-lock taper collet system. The principle of those chucks is that a collet formed substantially in the shape of a frusto-conical member exhibits an external taper in its axial length, which is compressed about a tool shank by being tightened with a sleeve, or nut. The prior art types of taper collet system chucks vary in gripping power.
Gripping power is an important, although not the only, measure of chuck performance. For chucks receiving tool shanks of 1.25" diameter the maximum gripping power force of the chuck is developed at an approximate depth of 1+inch within such chuck. For a taper collet milling chuck system of the conventional screw type the maximum gripping power is approximately 500 foot pounds. For a taper collet milling chuck system of the ballscrew type the maximum gripping power is approximately 1,000 foot pounds.
A desire to grip tools tighter in order that more torque may be transmitted thereto has resulted in tapered collet milling chucks which are improved over the conventional screw and ballscrew types. A conventional taper collet system roll-rock type milling chuck is capable of exerting an approximate 1,810 foot pounds of torque on a 1.25" diameter tool shank. A particular BIG-SHEPARD Gold-Power Milling Chuck (GMC) is capable of exerting a maximum gripping power of approximately 2,170 foot pounds. This particular high performance chuck employs a collet having a number of narrow and deep axial slots at the interior circumference of the collet. When a roll-lock type external sleeve is screwed upon the tapered external surface of the collet, the collet is compressed along its slots and very tightly grips the tool shank.
Although conventional system roll-rock type milling chucks and BIG-SHEPARD roll-lock type milling chucks both exhibit high gripping power, they suffer from some limitations. Howsoever strong the gripping power is within these chucks, even chucks employing slotted collets, when they are used to grip a completely cylindrical straight shank by compressive friction force then the gripping power developed will communicate less torque than is communicable through conventional side lock and threaded-shank tool holders. Therefore it would be desirable if the high torque transmission capabilities, and safety, of side lock and/or threaded-shak tool holders could somehow be combined with the ease of use of high gripping force chucks employing slotted collets.
In addition, the prior art tool holders or chucks typically do not ensure concentricity of the cutting tool within the tool holder, nor positively prevent tool twisting or axial movement within the tool holder. As such, cutting inaccuracies often develop during use which necessarily decrease machine production and efficiency. Furthermore, even should the principles of a side lock and/or a threaded-shank tool holder not prove to be amenable of combination with a roll-rock type or roll-lock type chuck employing a slotted collet, it would still be desirable to improve the manner in which the slotted collet of such a chuck is compressed. Particularly, in the conventional roll-rock type and the BIG-SHEPARD roll-lock type chucks there are approximately 300 needle roller rolls each approximately 0.08 inches diameter, which are each operating under compression loads as great as 90 pounds. The needle roller rolls are in such great multiplicities that they cumulatively permit forces in the order of 27,210 pounds in respect of the whole tightening nut. However, 90 pounds is tremendous individual stress upon these minute needle roller rolls, which correspondingly exhibit an abbreviated service life. Accordingly, it would be desirable if some means of compressing a slotted collet could be provided which did not place such large loads on such small bearings.
An improved chuck should have a large shrinkage allowance. A large shrinkage allowance is important to readily accommodate the differing tolerances in the shank diameters of tools, especially those made upon the shop floor. An improved chuck should exhibit high rigidity without excessive physical size and mass. The run out accuracy of a tool positioned within such a chuck should be good. The collet and gripping power of the chuck should not be adversely affected by a shank which is coated with oil. These requirements and others are addressed by the tool holder in accordance with the present invention.